Swift – How to use String.substringWithRange? (or, how do Ranges work in Swift?)

swift

I have not yet been able to figure out how to get a substring of a String in Swift:

var str = “Hello, playground”
func test(str: String) -> String {
 return str.substringWithRange( /* What goes here? */ )
}
test (str)

I'm not able to create a Range in Swift. Autocomplete in the Playground isn’t super helpful – this is what it suggests:

return str.substringWithRange(aRange: Range<String.Index>)

I haven't found anything in the Swift Standard Reference Library that helps. Here was another wild guess:

return str.substringWithRange(Range(0, 1))

And this:

let r:Range<String.Index> = Range<String.Index>(start: 0, end: 2)
return str.substringWithRange(r)

I've seen other answers (Finding index of character in Swift String) that seem to suggest that since String is a bridge type for NSString, the "old" methods should work, but it's not clear how – e.g., this doesn't work either (doesn't appear to be valid syntax):

let x = str.substringWithRange(NSMakeRange(0, 3))

Thoughts?

Best Answer

You can use the substringWithRange method. It takes a start and end String.Index.

var str = "Hello, playground"
str.substringWithRange(Range<String.Index>(start: str.startIndex, end: str.endIndex)) //"Hello, playground"

To change the start and end index, use advancedBy(n).

var str = "Hello, playground"
str.substringWithRange(Range<String.Index>(start: str.startIndex.advancedBy(2), end: str.endIndex.advancedBy(-1))) //"llo, playgroun"

You can also still use the NSString method with NSRange, but you have to make sure you are using an NSString like this:

let myNSString = str as NSString
myNSString.substringWithRange(NSRange(location: 0, length: 3))

Note: as JanX2 mentioned, this second method is not safe with unicode strings.

Using Objective-C Classes in Swift

If you have an existing class that you'd like to use, perform Step 2 and then skip to Step 5. (For some cases, I had to add an explicit #import <Foundation/Foundation.h to an older Objective-C File.)

Step 1: Add Objective-C Implementation -- .m

Add a .m file to your class, and name it CustomObject.m.

Step 2: Add Bridging Header

When adding your .m file, you'll likely be hit with a prompt that looks like this:

A macOS sheet-style dialog from Xcode asking if you would "like to configure an Objective-C bridging header"

Click Yes!

If you did not see the prompt, or accidentally deleted your bridging header, add a new .h file to your project and name it <#YourProjectName#>-Bridging-Header.h.

In some situations, particularly when working with Objective-C frameworks, you don't add an Objective-C class explicitly and Xcode can't find the linker. In this case, create your .h file named as mentioned above, then make sure you link its path in your target's project settings like so:

An animation demonstrating the above paragraph

Note:

It's best practice to link your project using the $(SRCROOT) macro so that if you move your project, or work on it with others using a remote repository, it will still work. $(SRCROOT) can be thought of as the directory that contains your .xcodeproj file. It might look like this:

$(SRCROOT)/Folder/Folder/<#YourProjectName#>-Bridging-Header.h

Step 3: Add Objective-C Header -- .h

Add another .h file and name it CustomObject.h.

Step 4: Build your Objective-C Class

In CustomObject.h

#import <Foundation/Foundation.h>

@interface CustomObject : NSObject

@property (strong, nonatomic) id someProperty;

- (void) someMethod;

@end

In CustomObject.m

#import "CustomObject.h"

@implementation CustomObject 

- (void) someMethod {
    NSLog(@"SomeMethod Ran");
}

@end

Step 5: Add Class to Bridging-Header

In YourProject-Bridging-Header.h:

#import "CustomObject.h"

Step 6: Use your Object

In SomeSwiftFile.swift:

var instanceOfCustomObject = CustomObject()
instanceOfCustomObject.someProperty = "Hello World"
print(instanceOfCustomObject.someProperty)
instanceOfCustomObject.someMethod()

There is no need to import explicitly; that's what the bridging header is for.

Using Swift Classes in Objective-C

Step 1: Create New Swift Class

Add a .swift file to your project, and name it MySwiftObject.swift.

In MySwiftObject.swift:

import Foundation

@objc(MySwiftObject)
class MySwiftObject : NSObject {

    @objc
    var someProperty: AnyObject = "Some Initializer Val" as NSString

    init() {}

    @objc
    func someFunction(someArg: Any) -> NSString {
        return "You sent me \(someArg)"
    }
}

Step 2: Import Swift Files to ObjC Class

In SomeRandomClass.m:

#import "<#YourProjectName#>-Swift.h"

The file:<#YourProjectName#>-Swift.h should already be created automatically in your project, even if you can not see it.

Step 3: Use your class

MySwiftObject * myOb = [MySwiftObject new];
NSLog(@"MyOb.someProperty: %@", myOb.someProperty);
myOb.someProperty = @"Hello World";
NSLog(@"MyOb.someProperty: %@", myOb.someProperty);

NSString * retString = [myOb someFunctionWithSomeArg:@"Arg"];

NSLog(@"RetString: %@", retString);

Notes:

If Code Completion isn't behaving as you expect, try running a quick build with ⌘⇧R to help Xcode find some of the Objective-C code from a Swift context and vice versa.
If you add a .swift file to an older project and get the error dyld: Library not loaded: @rpath/libswift_stdlib_core.dylib, try completely restarting Xcode.
While it was originally possible to use pure Swift classes (Not descendents of NSObject) which are visible to Objective-C by using the @objc prefix, this is no longer possible. Now, to be visible in Objective-C, the Swift object must either be a class conforming to NSObjectProtocol (easiest way to do this is to inherit from NSObject), or to be an enum marked @objc with a raw value of some integer type like Int. You may view the edit history for an example of Swift 1.x code using @objc without these restrictions.